In an image apparatus such as a video camera, a zebra pattern may be generated for indicating an over-luminance area of a picked up image in order to suggest a user to use a neutral density (ND) filter for reducing the intensity of light entering into an image sensor, thereby to achieve a high quality image. The over-luminance area is an area that has a luminance level exceeding a predetermined threshold level.
In general, the image apparatus can detect and select the over-luminance area based on a luminance detection, which may be implemented by hardware, software, or firmware. After that, a zebra pattern generator generates a zebra pattern to mask the over-illuminated area, based on information related to the selected over-luminance area. The operation and arrangement of the prior art technologies are further explained by a given example below.
In the conventional image apparatus, since the zebra pattern generator generates the zebra pattern and simply masks the over-illuminated area with this zebra pattern, the zebra pattern area displayed by such simple operations becomes discontinuous in a noisy environment or in a situation of a broader over-luminance threshold window, and thus has holes and noise patterns therein. In such case, the zebra pattern becomes unclear. To avoid such problem, some processes are required before generating the zebra pattern so as to generate a clearer area.
FIG. 1a to 1d are views explaining the operation for detection of an over-luminance area and for generation and display of a zebra pattern according to one example of a prior art video camera. As shown in FIG. 1a, an image frame 1 is the area picked up by the camera. In the frame 1, a mountain 2 is at the lower part of the captured area and has a low luminance level, the sky 3 has a luminance level relatively high but not higher than a threshold level, and a cloud 4 is an over-luminance area. In such case, an over-luminance detector finds that the cloud area 5 shown in FIG. 1b is over-illuminated. According to the information related to the cloud area 5, a zebra pattern generator generates a zebra pattern 6 as shown in FIG. 1c and overlays the cloud area 5 with the pattern 6 so as to generate a zebra pattern cloud area 7 as shown in FIG. 1d. White lines and black lines in the zebra pattern cloud area 7 may horizontally and vertically move for indicating the zebra pattern.
FIG. 5 shows one example of the arrangement of a prior art system for detecting luminance level and for generating and displaying a zebra pattern. As shown in FIG. 5, an input video signal 20 is input to the luminance detector 22. If the detector 22 detects an area having luminance level higher than the threshold, the information related to the detected area will be sent to a zebra generator 24 from the detector 22. The zebra generator 24 generates the zebra pattern and outputs it to a mixer 26. In the mixer 26, the video signal representing the pattern is overlapped with the input video signal 20 for generating a zebra pattern overlapping area to indicate the over-luminance area.
According to the prior art technologies, as shown in FIG. 2, the detected over-illuminated area becomes unclear as the noise level or the luminance level of the image becomes closer to the threshold. Therefore, holes or chunks may be generated in the detected over-illuminated area 8. One of the reasons causing such phenomenon may be due to the noise. Another reason is that the threshold used for the luminance detection is very closer to the luminance of some areas. The detected luminance level may randomly become higher or lower than the threshold because of little noises. This will generates holes or chunks in the detected over-luminance area. If such over-luminance information is used, holes or chunks will be generated in the zebra pattern 9, as shown in FIG. 2b, and the displayed zebra pattern will become unclear. In such situation, it becomes difficult for the user to understand or identify the zebra pattern, and the displayed zebra pattern is also unclear. In case of noises, the zebra pattern temporarily varies with noises in the original image.